Brainless means something stupid or foolish. That is enough to let you know that slots, dices and roulettes are not brainless (stupid or foolish) games.
What you can regard them as are games that requires no skills. Something fun may not require skills. Even most things that are fun do no require skills.
Does playing gambling games that require no skills make one dull? No.
You are right and there is no gambling games that should be considered brainless except the person has another definition of what they meant by brainless if not we have only understand one thing about being brainless and that's what you have already stated, perhaps OP wanted to use another word to describe his motion but ended up using this instead, or he might be thinking that those games that doesn't involves any thinking is a brainless gamble that is why he highlighted those games because in roulettes there is no calculation or any reliable strategy because is almost like spinning to be lucky for it to roll to the number you picked, so actually that's what I think was the reason why OP called it brainless.